The adjusting screw in my Campagnolo Veloce RD just broke off last week. It was probably 8 years old or so. Fortunately, I still had the derailer it had replaced (fell over, main body cracked), so I was able to swap the part out and keep rolling.
Other than corrosion, breaking, or wear, there's no reason to change a derailer. Just like a bike (or its rider!).